Photocycloaddition Reactions Research Articles

Insight for the photochemical reaction of 4-aryl-4H-pyran: Experimental and theoretical studies

In this article, we have investigated the photochemical reactions of 4-aryl-4H-pyran and found that the structural symmetry had a significant influence on its photochemical behavior. When symmetric 4-aryl-4H-pyran was used, such as diethyl 2,6-dimethyl-4-aryl-4H-pyran-3,5-dicarboxylate, mainly underwent [2 + 2] photocycloaddition reaction. While asymmetric 4-aryl-4H-pyran, such as 2-amino-3-cyano-4-aryl-4H-pyrans, mainly underwent photocleavage reactions. To speculate the reaction mechanism, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were carried out at the B3LYP-D3/def2TZVP//B3LYP-D3/6-31G(d) level. The results showed that diethyl 2,6-dimethyl-4-aryl-4H-pyran-3,5-dicarboxylate underwent a two-step [2 + 2] photocycloaddition to give 3,9-dioxatetraasterane derivatives via an intermolecular and an intramolecular reaction. And the 2-amino-3-cyano-4-aryl-4H-pyran underwent a ring cleavage to produce the conjugated diene derivatives. To explain the different photochemical behaviors of 4-aryl-4H-pyrans, electron spin density, frontier molecular orbitals (FMOs) analysis, electron-hole analysis and root-mean-square-deviation (RMSD) analysis were performed.

Kinetics and Mechanisms of Aqueous-Phase Reactions of Triplet-State Imidazole-2-carboxaldehyde and 3,4-Dimethoxybenzaldehyde with α,β-Unsaturated Carbonyl Compounds.

Reactions in the atmospheric aqueous phase are an important source of secondary organic aerosols (SOA). Within the present study, the reactions of triplet-state imidazole-2-carboxaldehyde (32-IC*) with methyl vinyl ketone (MVK, R1), methacrolein (MACR, R2), and methacrylic acid (MAA, R3), as well as the reaction of triplet-state 3,4-dimethoxybenzaldehyde (3DMB*) with the unsaturated compounds (MVK, R4), (MACR, R5), and (MAA, R6), in the aqueous phase were investigated using laser flash excitation-laser long path absorption and ultraperformance liquid chromatography coupled with high definition electrospray ionization spectrometry. The second-order reaction constants for 32-IC* were determined to be k1 = (1.0 ± 0.1) × 109 L mol-1 s-1 at pH 4-5 and 9, k2 = (1.4 ± 0.4) × 109 L mol-1 s-1 and (1.5 ± 0.1) × 109 L mol-1 s-1 at pH 4-5 and 9, and k3 = (1.4 ± 0.4) × 109 L mol-1 s-1 and (1.1 ± 0.4) × 108 L mol-1 s-1 at pH 4-5 and 9, respectively. The main products of the [2 + 2] photocycloaddition reactions of 32-IC* with both monomer and dimer of MVK as well as MACR were characterized. Similarly, the [2 + 2] photocycloaddition of the carbonyl of the excited triplet state of 3,4-dimethoxybenzaldehyde (3DMB*) with MVK was observed. The second order rate constants for the reactions of 3DMB* were determined: k4 = (1.5 ± 0.2) × 108 L mol-1 s-1, k5 = (2.8 ± 0.5) × 108 L mol-1 s-1, and k6 = (5.2 ± 1.2) × 106 L mol-1 s-1 at pH 9. The studied reactions show that different triplet photosensitizers react with strongly varying rate constants. Advanced CAPRAM process model studies show that active photosensitizers such as 3DMB* can quickly react with unsaturated organic compounds under deliquesced aerosol conditions modifying SOA, while the quenching with oxygen dominates the excited photosensitizer loss under cloud conditions.

Synthesis, Properties, and Intermolecular Interactions in the Solid States of π-Congested X-Shaped 1,2,4,5-Tetra(9-anthryl)benzenes

Abstract A Negishi coupling based synthesis of 1,2,4,5-tetra(9-anthryl)benzene derivatives, possessing X-shaped molecular structures, is described. The results of X-ray crystallographic analysis show that two-dimensional highly ordered packing structure of the crystalline state of the unsubstituted derivative is a consequence of intermolecular π-π and CH-π interactions between anthracene units. Photoirradiation of the unsubstituted derivative as a precipitated solution promotes intramolecular [4+4] photocycloaddition reactions between both adjacent pairs of anthracene units to produce a crystalline polycyclic product having a unique 1.700 Å long carbon-carbon single bond. Furthermore, charge-transfer complexes, displaying near-infrared absorption and emission, are generated by co-crystallization of the X-shaped unsubstituted member of the group with electron-acceptor molecules.

Competitive Processes of Energy Transfer and Opening of the Four-Membered Cycle in Cyclobutanes Formed in [2+2] Photocycloaddition Reaction of Bis(styrylbenzoquinoline) Dyads

Competitive Processes of Energy Transfer and Opening of the Four-Membered Cycle in Cyclobutanes Formed in [2+2] Photocycloaddition Reaction of Bis(styrylbenzoquinoline) Dyads

Arene Activation through Iminium Ions: Product Diversity from Intramolecular Photocycloaddition Reactions

AbstractWhile 2‐alk‐ω‐enyloxy‐sustituted benzaldehydes do not display any photochemical reactivity at the arene core, the respective iminium perchlorates were found to undergo efficient reactions either upon direct irradiation (λ=366 nm) or under sensitizing conditions (λ=420 nm, 2.5 mol% thioxanthen‐9‐one). Three pathways were found: (a) Most commonly, the reaction led to benzoxacyclic products in which the olefin in the tether underwent a formal, yet unprecedented carboformylation (13 examples, 44–99 % yield). The cascade process occurred with high diastereoselectivity and was found to be stereoconvergent. (b) If a substituent resides in the 3‐position of the benzene ring, a meta photocycloaddition was observed which produced tetracyclic skeletons with five stereogenic centers in excellent regio‐ and diastereoselectivity (2 examples, 58–79 % yield). (c) If the tether was internally substituted at the alkene, an arene photocycloaddition was avoided and an azetidine was formed in an aza Paternò–Büchi reaction (2 examples, 95–98 % yield).

Arene Activation through Iminium Ions: Product Diversity from Intramolecular Photocycloaddition Reactions.

While 2-alk-ω-enyloxy-sustituted benzaldehydes do not display any photochemical reactivity at the arene core, the respective iminium perchlorates were found to undergo efficient reactions either upon direct irradiation (λ=366 nm) or under sensitizing conditions (λ=420 nm, 2.5 mol% thioxanthen-9-one). Three pathways were found: (a) Most commonly, the reaction led to benzoxacyclic products in which the olefin in the tether underwent a formal, yet unprecedented carboformylation (13 examples, 44-99 % yield). The cascade process occurred with high diastereoselectivity and was found to be stereoconvergent. (b) If a substituent resides in the 3-position of the benzene ring, a meta photocycloaddition was observed which produced tetracyclic skeletons with five stereogenic centers in excellent regio- and diastereoselectivity (2 examples, 58-79 % yield). (c) If the tether was internally substituted at the alkene, an arene photocycloaddition was avoided and an azetidine was formed in an aza Paternò-Büchi reaction (2 examples, 95-98 % yield).

Photocontrollable Magnetism and Photoluminescence in a Binuclear Dysprosium Anthracene Complex.

By incorporating photoreactive anthracene moieties into binuclear Dy2O2 motifs, we obtain two new compounds with the formulas [Dy2(SCN)4(L)2(dmpma)4] (1) and [Dy2(SCN)4(L)2(dmpma)2(CH3CN)2] (2), where HL is 4-methyl-2,6-dimethoxyphenol and dmpma is dimethylphosphonomethylanthracene. Compound 1 contains face-to-face π-π interacted anthracene groups that meet the Schmidt rule for a [4 + 4] photocycloaddition reaction, while stacking of the anthracene groups in compound 2 does not meet the Schmidt rule. Compound 1 undergoes a reversible single-crystal-to-single-crystal structural transformation upon UV-light irradiation and thermal annealing, forming a one-dimensional coordination polymer of [Dy2(SCN)4(L)2(dmpma)2(dmpma2)]n (1UV). The process is concomitant with changes in the magnetic dynamics and photoluminescent properties. The spin-reversal energy barrier is significantly increased from 1 (55.9 K) to 1UV (116 K), and the emission color is changed from bright yellow for 1 to weak blue for 1UV. This is the first binuclear lanthanide complex that exhibits synergistic photocontrollable magnetic dynamics and photoluminescence. Ab initio calculations are conducted to understand the magnetostructural relationships of compounds 1, 1UV, and 2.

Total Synthesis of (+)-Cochlearol B by an Approach Based on a Catellani Reaction and Visible-Light-Enabled [2+2] Cycloaddition.

A 14‐step synthesis of (+)‐cochlearol B is reported. This renoprotective meroterpenoid features a unique core structure containing a densely substituted cyclobutane ring with three stereocenters. Our strategy employed an organocatalytic Kabbe condensation in route to the key chromenyl triflate. A subsequent Catellani reaction incorporated the remaining carbon atoms featured in the skeleton of cochlearol B. An ensuing visible‐light‐mediated [2+2] photocycloaddition closed the cyclobutane and formed the central bicyclo[3.2.0]heptane core. Notably, careful design and tuning of the Catellani and photocycloaddition reactions proved crucial in overcoming undesired reactivity, including cyclopropanation reactions and [4+2] cycloadditions.

Total Synthesis of (+)‐Cochlearol B by an Approach Based on a Catellani Reaction and Visible‐Light‐Enabled [2+2] Cycloaddition**

AbstractA 14‐step synthesis of (+)‐cochlearol B is reported. This renoprotective meroterpenoid features a unique core structure containing a densely substituted cyclobutane ring with three stereocenters. Our strategy employed an organocatalytic Kabbe condensation in route to the key chromenyl triflate. A subsequent Catellani reaction incorporated the remaining carbon atoms featured in the skeleton of cochlearol B. An ensuing visible‐light‐mediated [2+2] photocycloaddition closed the cyclobutane and formed the central bicyclo[3.2.0]heptane core. Notably, careful design and tuning of the Catellani and photocycloaddition reactions proved crucial in overcoming undesired reactivity, including cyclopropanation reactions and [4+2] cycloadditions.

Visible Light-Induced Regio- and Enantiodifferentiating [2 + 2] Photocycloaddition of 1,4-Naphthoquinones Mediated by Oppositely Coordinating 1,3,2-Oxazaborolidine Chiral Lewis Acid.

A range of asymmetric photochemical transformations using visible light have recently become considerably attractive. Among the various approaches, chiral Lewis acid association to enones for [2 + 2] and ortho photocycloadditions and oxadi-π-methane rearrangements have shown to be very promising. Naturally, chiral Lewis acid coordination protects one of the prochiral faces of the C═C double bond, which enables an effective enantiodifferentiation in the following bond-forming process(es). Here, we studied regio- and enantiodifferentiating [2 + 2] photocycloaddition reactions of naphthoquinone derivatives mediated by chiral oxazaborolidines. A stereochemical control was quite challenging for the 2-ene-1,4-dione substrate, as a double coordination of Lewis acid essentially cancels out the face selectivity, and a mono-coordination to each carbonyl group leads to an opposite stereochemical outcome. Furthermore, a stepwise coordination in the ground state of Lewis acid in a 1:1 fashion was practically inaccessible. We found that the excited-state decomplexation is a key to accomplish high regio- and enantioselectivities in the photocycloaddition of an ene-dione.

Keeping the name clean: [2 + 2] photocycloaddition.

Crossed [2 + 2] photocycloaddition is a specific case of intramolecular photocycloaddition reaction. Recently, the term "crossed [2 + 2] photocycloaddition" is interchangeably used to represent intermolecular [2 + 2] photocycloaddition reactions of two dissimilar double bonds/alkenes. To avoid confusion and to help researchers use the correct terminologies, this perspective clarifies the terminology used for different [2 + 2] photocycloaddition processes based on prior literature with the hope of establishing a standard for addressing the diverse set of photocycloaddition reactions thatwill be helpful to the chemical community.

Photoreactive Crystals Exhibiting [2 + 2] Photocycloaddition Reaction and Dynamic Effects.

ConspectusConducting a reaction in the solid state eliminates the usage of solvents. If such reactions are conducted in a single-crystal to single-crystal (SCSC) fashion, then structural characterization by single-crystal X-ray crystallography (SCXRD) techniques provides unequivocal structural details. Although topochemical principles govern, getting single crystals at the end of a SCSC reaction purely depends on the experimental skills of the researchers. SCSC reactions are common among solid-state [2 + 2] cycloaddition reactions (hereafter "photoreaction") after the classical work of Schmidt and co-workers in 1960s. Synthons and tectons in the crystal engineering box can be exploited to bring the functional groups into the required alignment and packing to achieve the desired chemical reactivities and physical properties, respectively. Bringing a pair of alkenes closer together in the organic molecules provides an effective starting point to achieve the goal of crystal engineering.Further, understanding and controlling photoreactivity in the solid state provide a gateway to designing new advanced materials, for example, making cycloreversible optical storage materials, photosalient and photomechanical materials, highly crystalline or even single-crystalline organic polymers, covalent organic framework structures, and organic polymers incorporated inside metal-organic frameworks (MOFs). Photoreactions often proceed in a SCSC manner due to the limited movements of the closely disposed reactive functional groups in the crystals. Thus, these photoreactions yield not only quantitative photoproducts but also regio- and stereospecificity, which are otherwise inaccessible by solution syntheses.The traditional definition of crystals being hard, rigid, and brittle is no longer valid ever since the mechanically responsive crystals were discovered. These dynamic crystals undergo various movements like curling, jumping, hopping, popping, splitting, and wiggling, when exposed to light (called "photosalient effect") or heat (called "thermosalient" effect). These crystals generate new methods of transforming light and heat energy into mechanical work. Recently, photosalient behavior during the [2 + 2] cycloaddition reaction under UV light has been frequently observed. With the emergence of the field of "crystal adaptronics", dynamic photoreactive crystals have emerged as smart actuating materials.This Account aims to provide an overview of the development in this area, since it has garnered much attention among solid state chemists. While presenting selected examples of important strategies, we try to illustrate the intentions and concepts behind the methods developed, which will help in a rational approach for the fabrication of advanced solid state materials. Apart from topochemical transformations, the important roles played by weak interactions, guest solvents, and mechanical grinding have been highlighted in several classes of compounds to show structural transformations that defy the expected outcomes. Overall, the progress of [2 + 2] cycloaddition reaction in solid state materials has been discussed from UV induced structural transformations to the development of smart actuating materials.

Intermolecular [2π+2σ]-photocycloaddition enabled by triplet energy transfer.

For more than one century, photochemical [2+2]-cycloadditions have been used by synthetic chemists to make cyclobutanes, four-membered carbon-based rings. In this reaction, typically two olefin subunits (two π-electrons per olefin) cyclize to form two new C-C σ-bonds. Although the development of photochemical [2+2]-cycloadditions has made enormous progress within the last century, research has been focused on such [2π+2π]-systems, in which two π-bonds are converted into two new σ-bonds1,2. Here we report an intermolecular [2+2]-photocycloaddition that uses bicyclo[1.1.0]butanes as 2σ-electron reactants3-7. This strain-release-driven [2π+2σ]-photocycloaddition reaction was realized by visible-light-mediated triplet energy transfer catalysis8,9. A simple, modular and diastereoselective synthesis of bicyclo[2.1.1]hexanes from heterocyclic olefin coupling partners, namely coumarins, flavones and indoles, is disclosed. Given the increasing importance of bicyclo[2.1.1]hexanes as bioisosteres-groups that convey similar biological properties to those they replace-in pharmaceutical research and considering their limited access10,11, there remains a need for new synthetic methodologies. Applying this strategy enabled us to extend the intermolecular [2+2]-photocycloadditions to σ-bonds and provides previously inaccessible structural motifs.

Engineering Photomechanical Molecular Crystals to Achieve Extraordinary Expansion Based on Solid-State [2 + 2] Photocycloaddition.

Photomechanical molecular crystals are promising candidates for photoactuators and can potentially be implemented as smart materials in various fields. Here, we synthesized a new molecular crystal, (E)-3-(naphthalen-1-yl)acrylaldehyde malononitrile ((E)-NAAM), that can undergo a solid-state [2 + 2] photocycloaddition reaction under visible light (≥400 nm) illumination. (E)-NAAM microcrystals containing symmetric twinned sealed cavities were prepared using a surfactant-mediated crystal seeded growth method. When exposed to light, the hollow microcrystals exhibited robust photomechanical motions, including bending and dramatic directional expansion of up to 43.1% elongation of the original crystal length before fragmentation due to the photosalient effect. The sealed cavities inside the microcrystals could store different aqueous dye solutions for approximately one month and release the solutions instantly upon light irradiation. A unique slow-fast-slow crystal elongation kinematic process was observed, suggesting significant molecular rearrangements during the illumination period, leading to an average anisotropic crystal elongation of 37.0% (±3.8%). The significant molecular structure and geometry changes accompanying the photocycloaddition reaction, which propels photochemistry to nearly 100% completion, also facilitate photomechanical crystal expansion. Our results provide a possible way to rationally design molecular structures and engineer crystal morphologies to promote more interesting photomechanical behaviors.

Tuning the photoreactivity of photocycloaddition by halochromism

Harnessing the power of light for chemical transformation is a long-standing goal in organic synthesis, materials fabrication and engineering. Amongst all photochemical reactions, [2 + 2] photocycloadditions are inarguably the most important and most frequently used. These photoreactions have green characteristics by enabling new bond formation in a single step procedure under light irradiation, without the need for heat or chemical catalysis. More recently, substantial progress has been made in red-shifting the activation wavelength of photocycloadditions in response to research trends moving towards green and sustainable processes, and advanced applications in biological environments. In the past 5 years, our team has further expanded the toolbox of photocycloaddition reactions that can be triggered by visible light. In our exploration of photochemical reactivity, we found that reactivity is often red-shifted compared to the substrate’s absorption spectrum. Our efforts have resulted in red-shifted photochemical reactions, providing some of the lowest energy – and catalyst-free – photo-activated [2 + 2] cycloadditions (up to 550 nm). More recently, we introduced an additional level of control over such finely wavelength gated reactions by altering the pH of the reaction environment, thus exploiting halochromic effects to enhance or impede the photoreactivity of red-shifted [2 + 2] photocycloaddition reactions. In this account, we discuss the current state of halochromically regulated photochemical reactions and their potential in soft matter materials on selected examples.

Enantioselective crossed intramolecular [2+2] photocycloaddition reactions mediated by a chiral chelating Lewis acid.

In intramolecular [2+2] photocycloaddition reactions, the two tethered olefins can approach each other in a straight or in a crossed fashion. Despite the fact that the latter reaction mode leads to intriguing, otherwise inaccessible bridged skeletons, there has so far not been any enantioselective variants thereof. This study concerned the crossed [2+2]-photocycloaddition of 2-(alkenyloxy)cyclohex-2-enones to bridged cyclobutanes. It was found that the reaction could be performed with high enantioselectivity (80–94% ee) under visible light conditions when employing a chiral rhodium Lewis acid as a catalyst (2 mol%).

2+2] Photocycloaddition in a bichromophoric dyad: photochemical concerted forward reaction following Woodward-Hoffmann rules and photoinduced stepwise reverse reaction of the ring opening via predissociation.

A novel biphotochromic dyad with styrylbenzo[f]quinoline photochromes was designed and synthesized to study the [2+2] photocycloaddition (PCA) reaction leading to cyclobutane with two benzo[f]quinoline (BQ) substituents and the reverse four-membered ring opening reaction. In the dyad, PCA occurs in a concerted manner according to the Woodward-Hoffmann rules in the ππ* excited state after excitation of the whole conjugated π-system comprising the ethylene group. Nanosecond time-resolved emission spectroscopy indicated formation of an excimer as a possible intermediate of the PCA reaction. The reverse reaction of photoinduced cyclobutane ring opening is assumed to proceed stepwise according to the predissociation mechanism: after excitation of the BQ substituent, energy transfer (ET) occurs from the bonding ππ* term localized on the BQ substituent to the dissociative πσ* term localized on cyclobutane; the efficiency of such a process was measured for the first time. For the first time, it is suggested that the predissociation mechanism is common to the ring-opening reaction of any cyclobutane with an unsaturated substituent where the π-system of the substituent rather than the σ-system of cyclobutane is excited under light irradiation.

Photoresponsive gadolinium–anthracene complexes: tuning the orientation and π–π stacking of anthracene groupsviaalkyl ester substituents of phosphonate ligands

[Gd(NO3)3(LR)2(dmpu)] containing 9-dialkyl phosphonomethylanthracene [LR, R = Me (1), Et (2), i-Pr (3)] show [4 + 4] photocycloaddition reaction for 1 and 2, but not for 3.

Mechanisms of the Cu(I)-Catalyzed Intermolecular Photocycloaddition Reaction Revealed by Optical and X-ray Transient Absorption Spectroscopies.

The [2 + 2] photocycloaddition provides a simple, single-step route to cyclobutane moieties that would otherwise be disfavored or impossible due to ring strain and/or steric interactions. We have used a combination of optical and X-ray transient absorption spectroscopies to elucidate the mechanism of the Cu(I)-catalyzed intermolecular photocycloaddition reaction using norbornene and cyclohexene as model substrates. We find that for norbornene the reaction proceeds through an initial metal-to-ligand charge transfer (MLCT) state that persists for 18 ns before the metal returns to the monovalent oxidation state. The Cu K-edge spectrum continues to evolve until ∼5 μs and then remains unchanged for the 50 μs duration of the measurement, reflecting product formation and ligand dissociation. We hypothesize that the MLCT transition and reverse electron transfer serve to sensitize the triplet excited state of one of the norbornene ligands, which then dimerizes with the other to give the product. For the case of cyclohexene, however, we do not observe a charge transfer state following photoexcitation and instead find evidence for an increase in the metal-ligand bond strength that persists for several ns before product formation occurs. This is consistent with a mechanism in which ligand photoisomerization is the initial step, which was first proposed by Salomon and Kochi in 1974 to explain the stereoselectivity of the reaction. Our investigation reveals how this photocatalytic reaction may be directed along strikingly disparate trajectories by only very minor changes to the structure of the substrate.

Emissive and reactive excimers in a covalently-linked supramolecular multi-chromophoric system with a balanced rigid-flexible structure

A novel multi-chromophoric system, triad, in which two styrylbenzoquinoline (SBQ) photochromes are connected by a balanced rigid-flexible linker comprising 2,3-naphthylene framework (a residue of 3-oxy-2-naphthoic acid) and tetramethylene groups, was designed and synthesized to study an excimer formation in the excited state. The 1H NMR data testified that triad exists in solution as folded conformers with asymmetric parallel-displaced SBQ units. Under light irradiation, in the triad, competitive photoisomerization and [2 + 2] photocycloaddition reactions were observed, both reactions being reversible. The photocycloaddition resulted in a tetrasubstituted cyclobutane. The red-shifted fluorescence spectrum and the appearance of a long-lived component in the triad fluorescence decay indicated formation of an 'emissive' excimer. The photocycloaddition is assumed to occur in a 'reactive' excimer, in which the ethylene groups of the SBQ photochromes are located at a distance sufficient for the formation of the σ-bonds between them. Quantum-chemical density functional theory (DFT) calculations at M06-2X/6-31G* level predicted the existence of the triad conformers with π-stacking interaction of SBQ photochromes, the structure of which is pre-organized for the excimer formation and photocycloaddition. For the first time, both emissive and reactive excimers were experimentally observed in the multi-chromophoric system with two diarylethylene photochromes undergoing [2 + 2] photocycloaddition.